refactor: extract Astrolabe to separate repository

Astrolabe has been extracted to its own repository at
github.com/cbcoutinho/astrolabe for independent releases.

Changes:
- Replace third_party/astrolabe/ directory with git submodule
- Remove astrolabe-ci.yml and appstore-build-publish.yml workflows
- Remove scripts/bump-astrolabe.sh
- Remove Astrolabe sections from bump-version.yml workflow
- Remove Astrolabe build steps from test.yml CI workflow
- Remove astrolabe volume mount from docker-compose.yml
- Simplify astrolabe install hook to always use app store
- Update CONTRIBUTING.md to reflect two-component monorepo

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -37,7 +37,6 @@ services:
       # Mount OIDC development directory outside /var/www/html to avoid rsync conflicts
       # The post-installation hook will register /opt/apps as an additional app directory
       #- ./third_party:/opt/apps:ro
-      - ./third_party/astrolabe:/opt/apps/astrolabe:ro
     environment:
       - NEXTCLOUD_TRUSTED_DOMAINS=app
       - NEXTCLOUD_ADMIN_USER=admin